Using "One Arduino, twelve piezo sensors, two speakers and a lot of duct tape" Alice Zappe and Julia Läger took an umbrella and converted it into a musical umbrella that turns falling raindrops into 8-bit sounds. For example:
Certainly something fun and a unique use of an Arduino board. There is a short wiki about the umbrella here, and it would seem that the piezos are being used as knock sensors to create values to generate sounds with.
To build embedded projects such as the musical umbrella, a great start is our Freetronics LeoStick, one of the smallest Arduino-compatible boards on the market - also with onboard RGB LED and piezo:
